Manufacture of metallic filaments for incandescent electric lamps.



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JAMES ALEXANDER SCOULAR, OF HAMMERSMITH, LONDON, ENGLAND, ASSIGNOR TO DICK, KERR & COMPANY LIMITED, OF LONDON, ENGLAND:

MANUFACTURE OF METALLIC FILAMENTS-FOR INCANDESCENT ELECTRIC LAMPS.

No Drawing.

T 0 all whom it may concern Be it known that 1, JAMES ALEXANDER SCOULAR, a subject of the King of England, residing at London, England, (whose postol'fice address is care of the Omega "Electric Lamp Company Limited, Rainville Road, Hammersmith, London, England,) have invented certain new and useful Improvements in the Manufacture of Metallic Filaments for Incandescent Electric Lamps, of which the following is a specification.

This invention relates to improvements in the manufacture of tungsten filaments for incandescent electric lamps and is a modification of the process described in the specification of my'prior patent entitled Manufacture of metallic filaments for electric incandescent.la mps,. No. 1104557, granted July 21, 1914. I

The process; described in the said prior specification is as follows:Tungsten powder is mixed with a small proportion of sodium silicate solution in the thick gelatinous or viscous 'form, the mixing being carried out in such a way as to effect a uniform incorporation of the powder with the binding agent. The resulting mass or paste is squirted through dies in any well known manner so to form raw filaments. These filaments are then subjected to heat so as to render them conductive and to cause them to sinter into finished metallic filaments. During this process the silicate is eliminated.-

()ne object of the present invention is to overcome the difiiculty that is met with in the manufacture of squirted filaments, by furnace treatment of a number of filaments at the same time, namely, that even after drying and the final sintering operations, a number of filaments frequently adhere together in bunches and cannot be separated, whereby whole batches of filaments are rendered useless.

According to this invention, in the manufacture of filaments for incandescent electric lamps of powdered metallic tungsten by the squirting process in which sodium silicate solution is used as a binding agent, the squirted filaments are treated after drying and baking with a solution of hydrofluoric acid.

On removing the filaments from the furnace in which the baking takes place, it is found that whole numbers of them adhere Specification of Letters Patent.

Patented Apr. 4, 1916.

together in bunches and cannot be easily" separated. At this stage, the bunches of filaments are treated with hydrofluoric acid whereby the bindin medium, which is, so

to speak, between tie filaments and causes their adhesion, is attacked by the acid and allows of easy separation of the filaments being effected. The filaments themselves consists of an agglomerated mixture of tungsten powder and water glass and are not affected by the hydrofluoric acid.,

One method of carrying the invention into effect will now be described. The raw filaments'are manufactured from tungsten powder with a. binding material of sodium silicate and squirted in the usual manner; the squirted filaments are hung over rods ofglass or Other material and placed in hot air ovens so as to straighten and dry the filaments. After drying, they are removed from the glass rods and placed in bundles on hooks of refractory metal, such for example as molybdenum or tungsten. The hooks from which the filaments are suspended are then placed in an electric furnace which is maintained at a bright red heat, the temperature being about 900 to 950C. The interior of the furnace is filled with dry hydrogen or other inert gas and the filaments supported on the hooks are introduced into the furnace so that they are subjected to uniform heating. Thereafter they are removed from the furnace and placed in a bath of commercial hydrofluoric acid (hot or cold) for about one hour, whereby the binding medium which causes the filaments to adhere together in bunches is eaten away; the bundles of filaments are rendered fairly flexible; they are then removed from the bath and washed with water for some time to remove excess of acid. If desired the filaments may remain in the Water until they are required for the final stage of the process. The filaments are then subjected to the final sintering operation by heating in a furnace which is maintained at a bright white heat, the temperature being about 1500 to 1600 C.

The time during which the filaments remain in the furnace depends upon their thickness, thicker filaments requiring a somewhat longer time in order to complete the sintering operation. The filaments on removal from the furnace are quite separate from one another and are then ready for mounting in the lamps.

The process for treating filaments as hcreinbet'ore described has reference to the manufacture of filaments furnaced in bulk, but it can also be usefully employed in the process in which the filaments are sintered separately. In such process the filaments either before or after being passed through the resistance furnace for the second time are separately mounted in clips and placed in a bell-jar which is exhausted, filled with hydrogen and again partially exhausted to leave a residual atmosphere of hydrogen. A current is then passed through each filament so as to bring the filament to a dazzling white heat as in the well known sintering operation. At this stage the filaments may be weighted so as to cause them to assume the desired final hair-pin shape.

What I claim as myinvention and desire to secure by Letters Patent is 1. A process for the manufacture of tungsten filaments for incandescent electric lamps, which consists in intimately mixing tungsten powder with sodium silicate solution to produce a paste, forming the paste into raw filaments, subjecting the filaments to heat to render them conductive and to cause them to sinter, immersing the filamersing the filaments in hydrofluoric acid,

washing the filaments in water to remove acid, and further heating the filaments in a resistance furnace in dry hydrogen.

3. The hereindescribed step in the process of manufacturing filaments for incandescent electric lamps from powdered metallic tungsten by the squirting process with a binding agent of sodium silicate solution consisting in immersing the baked filaments in a solution of hydrofluoric acid.

In testimony whereof I have signed my name to this specification in the presence of two subscribing witnesses.

JAMES ALEXANDER SCOULAR.

Witnesses:

H. D. JAMEsoN, O. J. WORTH. 

